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        document.write("<h1>九九乘法表（while版本）</h1>")
        var i=1
        while(i<=9){
            var j=1
            while(j<=i){ 
                document.write(j+"*"+i+"="+(j*i)+"&nbsp;&nbsp;")
                j++
            }
            document.write("<hr>")
            i++
        }

        document.write("<h1>九九乘法表（for循环版本）</h1>")
        for(var x=1;x<=9;x++){
            for(var y=1;y<=x;y++){
                document.write(y+"*"+x+"="+(x*y)+"&nbsp;&nbsp;")
            }
            document.write("<hr>")
        }

        var arr=["北京","上海","广州"]
        document.write("<ul>")
        for(var index=0;index < arr.length;index++){
            document.write("<li>"+arr[index]+"</li>")
        }
        document.write("</ul>")

        document.write("<ul>")
        for(var index in arr){
            document.write("<li>"+arr[index]+"</li>")
        }
        document.write("</ul>")

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